The invention relates to the field of hydrodynamic bearing assemblies of the type that cooperates with high-speed spindle elements. More specifically, the invention relates to a design that reduces stress and distortion in parts which are incorporated into a hydrodynamic bearing.
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside a hub that supports the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball or hydrodynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets cause the spindle to rotate, thereby rotating the disc and passing the information storage tracks beneath the head.
The use of hydrodynamic bearing assemblies in such drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In hydrodynamic bearings, a lubricating fluid, such as oil or air, functions as the bearing surface between a base or housing and a spindle or hub. As the lubricating fluids require small gaps between the stationary and rotating members in order to provide the support, stiffness and lubricity required for proper bearing operation, conventional drive components and assemblies typically require tight tolerances and demand precision assembly methods. Such demanding tolerance and assembly control results in increased part and assembly costs along with an increased level of quality control to ensure proper drive operation.
Thus, the problem presented is to reliably set close bearing gaps without requiring excessive or burdensome part or manufacturing tolerances.
However, a related problem arises from the fact that to maintain the integrity of the thrust plate style hydrodynamic bearing, the technology has adopted the approach of laser welding the counterplate which overlies the thrust plate and is supported adjacent the thrust plate an upraised wall of the sleeve. This seals the fluid dynamic bearing, maintaining the fluid within the bearing without the necessity of using an o-ring or the like between the counterplate and sleeve to prevent any loss of fluid. However, thermal contraction forces caused by cooling of the weld nugget cause the counterplate to bow (typically outward), which may increase the gap between thrust plate and counterplate and thereby impact stiffness and wear contact conditions for the thrust bearing. Therefore, the problem presented is to adopt a design which eliminates or diminishes the bowing of the counterplate during welding.
The present invention seeks to provide a method and apparatus for eliminating bowing or other distortion of the counterplate incorporated into a thrust plate design hydrodynamic bearing.
More specifically, the present invention seeks to improve the method of welding the counterplate to the surrounding supporting sleeve by making the effect on the counterplate substantially distortion free.
In summary, according to the present invention, the counterplate and/or sleeve are modified to reduce or eliminate distortion in the counterplate in a laser welded fluid bearing design. More specifically, according to the present invention, which comprises a shaft with a thrust plate at one end, surrounded by a sleeve, and a counterplate supported in the sleeve adjacent the thrust plate to define a gap with the thrust plate. It is known to directly weld the counterplate which lies across the end of the shaft and the thrust plate to a wall or shoulder of the sleeve in which the counterplate is fit. According to the invention, the radial stiffness of the sleeve is weakened by cutting grooves or the like at or near the interface between the sleeve wall and the counterplate, or on the outer diameter of the sleeve wall to essentially weaken the sleeve wall. This weakening of the sleeve wall prevents bowing or distortion of the counterplate being imposed by the sleeve.
More specifically, in one embodiment, the distortion is eliminated by cutting a groove at the interface of counterplate and sleeve. The groove may be solely in the sleeve shoulder or may also intrude into the counterplate. In a preferred embodiment, the depth is about half of the counterplate thickness.
In yet another alternative embodiment, a groove is cut on the sleeve outer diameter inward toward the counterplate, approximately radially parallel with the counterplate. In yet another approach, the sleeve may simply cut away at the axially or grooved and radical outer end of the shoulder which supports the counterplate.
In yet a further alternative embodiment, an undercut may be imposed on the inner surface of the sleeve shoulder at or near where a corner of the counterplate would rest against the sleeve shoulder.